Glass-ceramics composed of lithium silicate glass have been proposed for use as dental restorative materials. For instance, see Barrett et al., U.S. Pat. No. 4,189,325 and reference cited therein. The patentees employ a mixture of elemental platinum and niobium oxide as nucleating agents to eliminate the problem of cracking caused by localized volume changes during crystallization. At col. 4, lines 55-59, the patentees state that "conventional nucleating agents such as TiO.sub.2, ZrO.sub.2 or P.sub.2 O.sub.5 . . . " need not be used.
In accordance with this invention, we have found that if one employs P.sub.2 O.sub.5 as a nucleating agent in lithium silicate glasses similar to those described by Barrett et al., in place of the patentees' Pt--Nb.sub.2 O.sub.5 nucleating agents, a substantial and unexpected improvement in strength is obtained, along with the ability to attain concomitant high softening temperatures, which enhances the utility as a dental restorative material because the addition of a glaze to the dental restoration is thereby rendered less critical.